The present invention relates to a fluid flow device and liquid metering, and more particularly to a method and apparatus for producing a combustible air-liquid fuel mixture having a substantially constant air-to-fuel ratio.
U.S. Pat. No. 3,778,038, granted Dec. 11, 1973, explains a method and apparatus for producing a uniform combustible mixture of air and minute liquid fuel droplets for delivery to the intake manifold of an engine. The apparatus includes an intake air zone connected to a variable area throat zone for constricting the flow of air to increase its velocity to sonic. Liquid fuel is introduced into the air stream at or before the throat zone to minutely divide and uniformly entrain fuel as droplets in the air flowing through the throat zone. Walls downstream of the throat zone are arranged to provide a gradually increasing cross-sectional area for efficiently converting a substantial portion of the kinetic energy of the high velocity air and fuel to static pressure. Such conversion enables the maintenance of sonic velocity air through the throat zone over substantially the entire operating range of the engine to which the air-liquid fuel mixture is supplied.
The above U.S. patent further explains the well known phenomena that under sonic conditions, the pressure of the air stream at the throat zone is approximately 53% of atmospheric pressure. Under sonic conditions and when the atmospheric pressure remains constant, it is possible to provide an air-liquid fuel mixture having a substantially constant air-to-fuel ratio by simply metering the rate of fuel delivered into the air stream in direct proportion to the area of the throat zone. However, when atmospheric pressure varies, possibly due to altitude changes, the mass rate of air flowing through the apparatus also varies. Also, atmospheric temperature conditions have a bearing upon the mass rate of air flowing through the apparatus. When these atmospheric changes occur it is necessary to adjust the rate of fuel introduced into the high velocity air stream in order to maintain a substantially constant air-to-fuel ratio. For example, when atmospheric pressure decreases, the air passing through the apparatus is somewhat thinner and it has less mass density when compared to air at a higher pressure. Accordingly, less fuel is required to produce a mixture having the same air-to-fuel ratio as before the atmospheric presure change. A fuel metering system which relies solely upon the area of the throat zone or the volume of air passing therethrough does not adjust or otherwise correct for such atmospheric fluctuations, and the air-to-fuel ratio varies depending upon atmospheric conditions.
It is also desirable that the apparatus for producing the air-liquid fuel mixture be relatively insensitive during the critical idling mode of the engine for which the mixture is produced. In other words, the structure for varying the area of the throat zone should be capable of easily achieving such variation in very small increments. Drastic area changes during the idling mode result in equally drastic changes in the quality and quantity of the air-liquid fuel mixture produced. Moreover, the geometry of the apparatus must be such that the maximum air flow rate required by the engine is provided without adversely affecting efficient conversion of the kinetic energy of the high velocity air and fuel to static pressure. It is highly desirable to maintain sonic air flow at the throat zone over substantially the entire operating range of the engine, even when the engine requires its maximum volumetric air flow rate. Such criteria may require dual passageways, as explained below.
Finally, it is desirable to provide a relatively constant pressure signal for drawing fuel into the high velocity air stream while maintaining sonic flow at the throat zone over substantially the entire operating range of the engine. Constant vacuum carburetors of the type described in 1 Fisher, Carburation, 4th Ed. 1963 and Larew, Carburetors & Carburetion, 1967 attempt to provide a constant pressure signal for fuel introduction but these devices are complex and the velocity at the butterfly valve varies throughout most of the operating cycle of the engine.